The present invention relates to radiation-curable coating compositions based on tin-free aqueous polyurethane dispersions, a process for the preparation thereof, the use of the coating compositions as lacquers and/or adhesives, and objects and substrates provided with these lacquers and/or adhesives.
Aqueous radiation-curable polyurethane dispersions have hitherto been synthesized via tin-catalysed urethanization. The main reason for the catalysis is that during synthesis of the polyurethane temperatures of 70° C. should not be substantially exceeded, in order to avoid thermally initiated polymerization of the unsaturated groups which can undergo free radical polymerization. At such low temperatures, aliphatic isocyanates such as are preferably used in aqueous radiation-curable polyurethane dispersions react only very slowly. Dibutyltin dilaurate (DBTL) is by far the preferred catalyst for synthesis of aqueous radiation-curable polyurethane dispersions because of the high selectivity for the OH-NCO addition and the high catalytic activity.
For the catalysis of the urethanization, various other metal and non-metal catalysts, such as e.g. tertiary amines, compounds of tin, zinc, zirconium, copper, bismuth, titanium and molybdenum, are known from lacquer application of one-component (1C) and two-component (2C) polyurethane dispersions, i.e. the reaction of hydroxy-functionalized polyurethane dispersions with blocked and non-blocked polyisocyanates by stoving on the substrate. These catalysts are optimized for the preparation of 1C or 2C lacquers, i.e. they are said e.g. to have similar pot lives and temperature activity profiles to DBTL or to prefer the isocyanate-alcohol reaction over the isocyanate-water reaction. For the synthesis of the polyurethane of an aqueous radiation-curable polyurethane dispersion, they are unsuitable and significantly inferior to DBTL because of side reactions (e.g. allophanation), a low catalytic activity under the reaction conditions typical of the synthesis of aqueous radiation-curable polyurethane dispersions, or because of the property that too low molecular weights are achieved.
Aqueous radiation-curable polyurethane dispersions which are prepared with unsuitable catalysts show a coarser particle pattern, sediment immediately or are significantly more highly viscous than aqueous radiation-curable polyurethane dispersions of the same composition which are catalysed with DBTL.
Q. Bell, Raw Materials and their Usage, in: Solvent-Borne Urethane Resins, vol. 1: Surface Coatings, Chapman and Hall, New York, 1993, p. 153 et seq. describes various aminic and metal-based catalysts which catalyse the OH-NCO addition in 2C uses. The suitability of these catalysts for synthesis of the polyurethane from aqueous radiation-curable polyurethane dispersions is not described.
WO 2008148739 A1 describes various catalysts which are suitable in principle for the preparation of the polyurethane of an aqueous radiation-curable polyurethane dispersion. DBTL is clearly preferred, and also used in the examples.
DE 102007006492 A1 and EP 753531 A1 describe various catalysts which are suitable in principle for the preparation of the polyurethane of an aqueous radiation-curable polyurethane dispersion. DBTL is clearly preferred, and also used in the examples.
There was a general need to change radiation-curable aqueous polyurethane dispersions for the preparation of wood lacquers over to novel lacquers which contain no organotin compounds. A representative example of such a requirement are the specifications required for coatings by IKEA in IOS-MAT-066, 2006, p. 4.
There was the object of providing alternative tin-free polyurethanes for aqueous radiation-curable polyurethane dispersions to the existing DBTL-catalysed polyurethanes for aqueous radiation-curable polyurethane dispersions. In this context, the properties of the aqueous radiation-curable polyurethane dispersions should not deviate from those of aqueous radiation-curable polyurethane dispersions synthesized via DBTL catalysis.
It has been found, surprisingly, that bismuth salts in the presence of acids having a pKa of <2.5 are outstandingly suitable for synthesizing polyurethane acrylates for aqueous radiation-curable polyurethane dispersions. The polyurethane acrylates for aqueous radiation-curable polyurethane dispersions which are catalysed in this manner correspond in physical and use properties to the DBTL-catalysed polyurethane acrylates for aqueous radiation-curable polyurethane dispersions. It is furthermore the object of a preferred embodiment of this invention to establish the weight-average molecular weight MW of the polyurethane acrylate in a range of from 103 to 106 g/mol. In this context, weight-average molecular weights MW of polyurethane acrylates such as are achieved for DBTL-catalysed systems can be achieved.